Ink jet printing is a technology by which ink is applied to a substrate through jet nozzle located at the print head of ink-jet printer. Ink-jet printers may generally be divided into two types, based on the mode by which ink is dispensed through the print head. One type is known in the art as continuous ink jet printers, in which ink is being distributed from the print head in a continuous stream, which is then broken into uniform droplets that would later be directed to their final destination. The second type of ink jet printers is known in the art as drop on demand. In this type of printers a drop is created and ejected from the print head on a given cue only if it is to be applied to a substrate. The basic ingredients of ink composition are vehicle, colorant and additives, a combination of which gives the ink its unique properties. Various kinds of ink compositions exist for wide a range of printing purposes and for a variety of substrate requirements. For example, inks may be water based, inks may be based on volatile organic solvents or non volatile oils. Another kind of ink jet ink is UV-curable ink.
UV curable ink is composed of such chemical components that allow it to polymerize and solidify in response to irradiation, hence cure on a substrate. A typical UV curable ink composition may include a combination of chemical components such as: photoinitiators, monomers, oligomers, colorants, diluents, resins and surfactants.
The typical and more common irradiation source for UV-curable inks is UV light source. However, depending upon the ink ingredients, it may also be cured by irradiation using other energy sources, such as electron bean. The curing process is a chemical reaction in which the activated monomers and oligomers ingredients of the ink polymerize to produce solid ink that is cured on a substrate. The polymerization reaction is initiated by irradiation of the ink that has been applied to a substrate, generally by using a UV light source, leading to photo-activation of the photoinitiators components of the ink mixture. These activated photoinitiators may now activate the monomers and oligomers of the ink composition and as a result a polymerization reaction may proceed. Depending on the ingredients of the ink composition, UV-curable inks exhibit varying degrees of viscosity at room temperature. Ink-Jet printing requires low viscosity inks for jetting, but higher viscosity is essential for controlling drops on the printed surface.
The colorant ingredients within the ink composition determine the color properties of the ink. The two main groups of colorants are dyes and pigments. Dyes are substances that are soluble in ink vehicle. They are less frequently used in UV-curable inks due to their low resistance to UV radiation and can mainly be found in water or solvent based inks designed for indoor applications. Pigments are small particle of colorants that are non soluble in the ink vehicle. They usually have higher resistance to UV light and therefore are the more frequently used colorants in UV-based inks and in solvent or water based inks for outdoor applications. Pigments are usually low-soluble small crystals that may be divided into two subgroups, based upon their chemical structure: non-organic pigments that are mainly used in achromatic (colorless) inks; and organic pigments, which account for colored inks.
Some of the advantages of using UV-curable ink include: safer to use because they exhibit no or very low Volatile Organic Compound (VOC) emissions, fast drying, low energy requirements and excellent mechanical properties, which allow them to be used on a wide range of substrates, in particular non-porous substrates such as plastic, glass, metal. However, there are also several disadvantages when using the current forms of UV-curable ink: when the UV inks are jetted onto porous substrates, the ink can penetrate the substrate, resulting in poor image quality, diluted color and bleed-through. Furthermore, depending upon the ink composition, additional instrumentation may be included to allow the liquefaction (for example by heating) of the ink composition prior to being jetted from the printer. In addition to increasing the costs of using the UV-curable ink printer, the printing speed is also limited. Another disadvantage of using the current compositions of UV-curable inks is the relatively high costs of these inks, attributed mainly to the high cost of ink ingredients, such as monomers, oligomers and photoinitiators. There is thus a need in the art for new ink compositions, methods of printing and printing devices that would improve the printing efficiency and cost.